Late-season weed escape survey in Wisconsin identifies a second county with a glyphosate-resistant horseweed population

Ross Recker (Graduate Research Assistant), John Buol (Undergraduate Research Assistant), and Vince Davis (Assistant Professor) Department of Agronomy, UW-Madison

Glyphosate-resistant weeds continue to be a major threat to corn and soybean production across the Nation. A pro-active survey of late-season weed escapes in corn and soybean fields was conducted throughout Wisconsin in 2012 and 2013. One objective of the survey was to identify areas where populations of glyphosate-resistant weeds may exist.

In 2012, a population of horseweed (Conyza Canadensis) collected from Jefferson County, and glyphosate screening in the greenhouse confirmed it was glyphosate-resistant with a 6-fold tolerance compared to a susceptible population. This announcement and more information about horseweed biology and management were previously reported and can be read here.

During the 2013 late-season survey, another putative glyphosate-resistant horseweed population was collected from Columbia County. Again whole-plant dose response experiments were recently conducted in the greenhouse and results confirm this population is also nearly six-fold glyphosate-resistant (Figure 1).

Figure 1. Shoot dry biomass of Columbia County horseweed and susceptible horseweed following treatment with glyphosate at doses up to 3.48 kg ae ha-1 as estimated by a four-parameter log-logistic regression function.

Figure 1. Shoot dry biomass of Columbia County horseweed and susceptible horseweed following treatment with glyphosate at doses up to 3.48 kg ae ha-1 as estimated by a four-parameter log-logistic regression function.

The horseweed seeds were collected from mature plants in a soybean field with a history of no-till, corn/soybean rotation, and glyphosate use. These plants displayed symptomology at the time of collection including shortened internodes (Figure 2) and were scattered randomly at low densities in about a 2 to 3 acre patch. If you have horseweed or other weeds that survive postemergence applications, you should have concern about herbicide resistance. Contact your local county Ag Extension Agent who can help you further evaluate the situation and plan a pro-active resistant management program so you can take action against herbicide resistance.

Figure 2. Horseweed plant late in the 2013 growing season that was not controlled with a previous postemergence glyphosate application in a no-till soybean field in Columbia County, Wisconsin.

Figure 2. Horseweed plant late in the 2013 growing season that was not controlled with a previous postemergence glyphosate application in a no-till soybean field in Columbia County, Wisconsin.

Updated 2013 WCWS Research Report


The 2013 WCWS Research Report is now updated with nine corn and seven soybean herbicide evaluation trials. Some highlights include:

  • Redroot pigweed and wild buckwheat efficacy data on pages 19 to 31.
  • Liberty Link system comparisons of PRE+EPOST, PRE+MPOST, and EPOST+MPOST programs on pages 81 to 91.
  • Dandelion control with preplant-incorporated herbicides on pages 1 to 6.
  • Comparisons of corn PRE only, PRE_POST, and POST only programs on pages 7 to 13 and 48 to 55.

New USDA Cover Crop Termination Guidelines

The NRCS just released new cover crop termination guidelines. Recently, a webinar was held to discuss the new changes and answer questions. To access the archived webinar, “Cover Crops and Crop Insurance: Questions and Answers on USDA’s Cover Crop Termination Guides”, follow this link to the YouTube video. Most of the changes involve the termination guidelines for areas of the country with a summer fallow practice. The termination zone for Wisconsin is Zone 4 and the guidelines have not changed from the June 2013 to the December 2013 guideline release. To see the new termination guidelines released in December 2013 from the USDA-NRCS, go here. For the previous termination guidelines released in June 2013, go here.

In addition, the USDA’s Risk Management Agency just published a new cover crops fact sheet outlining their policy from a crop insurance standpoint. To access their fact sheet, go here. According to the RMA, a cover crop can be harvested for forage or grazed without violating your crop insurance policy. However, it is important to remember that when removing a cover crop as a forage crop the herbicide label restrictions must be followed when feeding to livestock. Also, removing biomass for forage may limit the benefits that you may expect for a cover crop. Researchers looking at crimson clover managed as a cover crop with no removal versus a spring forage harvest found that corn grain and yields were higher in the fields with the cover crop compared to the forage crop 1. However, in another study with cereal rye (Secale cereale) in a corn grain system, removal of the aboveground rye biomass did not affect the subsequent corn crop yield 2. Aside from yield, removing biomass for forage can impact soil health. In a 5 year study of three cover crops in a corn grain system, removal of the cover crop biomass reduced soil organic carbon and nitrogen 3.


  1. Holderbaum, J.F.; Decker, A.M.; Meisinger, J.J.; Mulford, F.R.; Vough, L.R. Harvest management of a crimson clover cover crop for no-tillage corn production. 1990 Agronomy Journal 82(5): 918-923.
  2. Tollenaar, M.; Mihajlovic, M.; Vym, T.J. Corn growth following cover crops: influence of cereal cultivar, cereal removal, and nitrogen rate. 1993 Agronomy Journal 85(2): 251-255.
  3. Kuo, S.; Jellum, E.J. Influence of winter cover crop and residue managment on soil nitrogen availability and corn. 2002 Agronomy Journal 94(3): 501-508.

Palmer amaranth identified through the late-season weed escape survey

Vince Davis (Assistant Professor) Department of Agronomy, UW-Madison; Ross Recker (Graduate Research Assistant)

Palmer amaranth (Amaranthus palmeri) is a dioecious, summer annual broadleaf weed species in the pigweed (Amaranthaceae) family that is extremely adaptable to environments, including the development of herbicide resistance, and it is extremely competitive with row crops1. Palmer amaranth has been tormenting cotton and soybean producers in the southeast United States for the past decade, and more recently Palmer amaranth has been moving its way north into states such as Iowa2,3, Illinois4, Indiana5, and Michigan6,7,8. This northward movement of Palmer amaranth is alarming, and the movement has often been attributed to spreading contaminated manure from animal production operations that have fed cottonseed feed by-products transported from Southern U.S. production fields, as well as equipment movement, and contaminated seed for Prairie restorations.

Palmer amaranth is not native to Wisconsin. A population was identified in Dane County, WI through the 2013 late-season weed escape survey efforts partially funded by the Wisconsin Corn Promotion Board. During this survey in fall 2013, five plants were distantly distributed in a large soybean field. Four of those plants were male plants (Figure 1), and luckily only one plant was a female plant was present and produced minimal seed in comparison to the seed production they can potentially produce (Figure 2).

Figure 1. Male Palmer amaranth plant from Dane County, WI


Figure 2. Female Palmer amaranth plant from Dane County, WI


Plant tissue from all five plants was sent to the Dr. Pat Tranel at the University of Illinois. Dr. Tranel’s lab conducted molecular techniques to confirm that these plants were in fact Palmer amaranth as well as quantify the number of copies of the EPSPS gene. All five plants were confirmed as Palmer amaranth, and subsequently they produced EPSPS gene amplification ranging from 3-fold to >20-fold. EPSPS gene amplification within those ranges has previously demonstrated to be an effective mechanism for evolved glyphosate resistance in Palmer amaranth9. Whole-plant dose response experiments will be conducted to further confirm if this plant population is in fact resistant to glyphosate, but the molecular findings are at this point a strong indication that it is likely resistant. The origin of how this population established in Dane county, like many others, is difficult to pinpoint, but these plants were found in a field with a history of dairy manure application.

Because this is so far only one confirmed location of questionably resistant Palmer amaranth with minimal plants at this location, this does not necessarily represent a wide-spread catastrophe. However, this does provide further indication that the threat of herbicide-resistant pigweeds in Wisconsin crop production is real. The best approach is to be aware of this threat and implement a robust Integrated Pest Management approach, if you’re not already doing so. This approach should start with intently scouting fields and identifying weeds this spring prior to preplant control. Utilize diverse preplant control methods to ensure starting with a clean field at planting, but make sure scouting and proper identification is done prior to postemergence herbicide applications. Consider interrow cultivation and tank-mix herbicides that provide a second effective mode-of-action for key weed species that need controlled in-crop. Most importantly, intently scout following postemergence applications to look for weeds that were not controlled. Those are the weeds that pose the biggest threat to building a population with herbicide resistance. If any of these scouting trips indicates a pigweed species is one of the main target weeds, then make certain you know what type of pigweed species it is. Redroot pigweed, smooth pigweed, Powell amaranth, and waterhemp are all common pigweed species, however, waterhemp poses the greatest risk of herbicide resistance from that list. As already mentioned Palmer amaranth is not a native, or common, pigweed species, but it poses a significant risk. If Palmer amaranth is identified, then its presence should invoke a “zero tolerance” mindset with eradication as the goal where feasible.

Characteristics of Palmer amaranth include rapid growth rate, high seed production, high degree of genetic diversity, high water use efficiency, and rapid development of herbicide resistance. Palmer amaranth competition with crops has demonstrated yield losses as high as 78% (soybean) and 91% (corn)10,11. Therefore, Palmer amaranth should be of high concern for producers across the state. Below are links to help with the identification and management of Palmer amaranth. Key identification points are: the stems lack hair (like waterhemp, but different than Powell amaranth, redroot pigweed, and smooth pigweed), petioles are often longer than the leaf blade (Figure 3), a long terminal seed head, and seed heads of female plants are very prickly and painful to grab with a bare hand.

Figure 3. Comparison of Palmer amaranth petiole length to leaf blade length


Because this weed has caused such a problem in our surrounding states over the recent couple of years, there are many helpful extension articles we have referenced in this article. Specifically, here are some helpful guides for identification:

  • Identification of the weedy pigweeds and waterhemps of Iowa. D.B. Pratt, M.D.K. Owen, L.G. Clark, and A. Gardner. 1999. Available at: Iowa State University Extension
  • Guidelines for the identification and management of Palmer Amaranth in Illinois Agronomic Crops. A. Hager; available at: University of Illinois IPM Bulletin
  • Palmer amaranth biology, identification, and management. T. Legleiter and B. Johnson. Available at: Purdue University Extension
  • Identifying Palmer amaranth in the field — Video. B. Johnson and T. Legleiter. YouTube video
  • Palmer amaranth in Michigan, Keys to Identification. C. Sprague, C. Michigan State University Weed Science. Available at: Michigan State University

If you or your crop scout has utilized the identification guides and believe you have Palmer amaranth escapes in your fields, please contact your local county extension agent and/or Dr. Vince Davis ( or (608) 262-1392.


  1. Ward, S.M., T. M. Webster, L. E. Steckel. 2013. Palmer Amaranth (Amaranthus palmeri): A review. Weed Tech. 27:12-27.
  2. Hartzler, B. and M. Owen. 2013. Troublesome Palmer amaranth expanding its range. Iowa State University, Integrated Crop Management News: article is available here
  3. Hartzler, B. 2013. Palmer amaranth update. Iowa State University, Integrated Crop Management News: article is available here
  4. Hager, A. 2013. Update on Palmer amaranth distribution in Illinois. University of Illinois, The Bulletin: article is available here
  5. Johnson, B. and T. Legleiter. 2013. Palmer amaranth confirmed in 17 Indiana counties. Purdue Agriculture News. Available at: Purdue University
  6. Sprague, C. 2012. Palmer amaranth found in more Michigan fields: Now is a good time to scout. Michigan State University Extension News. Available at: Michigan State University
  7. Sprague, C. 2013. Palmer amaranth: Why this weed should alarm you. Michigan State University Extension News. Available at:
  8. Sprague, C. Palmer amaranth: A new invasive weed to watch for in Michigan. Available at:
  9. Gaines, T.A., W. Zhang, D. Wang, B. Bukun, S.T. Chisholm, D.L. Shaner, S.J. Nissen, W.L. Patzoldt, P.J. Tranel, A.S. Culpepper, T.L. Grey, T.M. Webster, W.K. Vencill, R.D. Sammons, J. Jiang, C. Preston, J.E. Leach, and P. Westra. 2010. Gene amplification confers glyphosate resistance in Amaranthus palmeri. PNAS 107:1029-1034.
  10. Bensch, C.N., M.J. Horak, and D. Peterson. 2003. Inference of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp (A. rudis) in soybean. Weed Sci. 51:37-43.
  11. Massinga, R.A., R.S. Currie, M.J. Horak, and J. Boyer. 2001. Interference of Palmer amaranth in corn. Weed Sci. 49:202-208

Herbicide Resistant Horseweed in Wisconsin

In a recent issue of the Wisconsin Crop Manager, Ross Recker, Dave Stoltenberg, and Vince Davis reported an incidence of horseweed, Conyza canadensis L., resistant to glyphosate in Wisconsin.

To read the article, go here.

Or, to download the Wisconsin Crop Manager from this website, click here.